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Preventing faults before they occur is a very important factor influencing the cost and time of replacement and maintenance of machines or their components. It is much easier to plan such actions than to react to sudden failures. A vibrometer is a useful device for precise measurements that facilitate diagnostics and the inspection of wear and tear of components. In this article, we will discuss how vibrometers work and present several popular models.
Vibration diagnostics, i.e. vibration and oscillation testing, is one of the fundamentals of the mechanical diagnostics of machines. It is indispensable when evaluating the balance of a machine and its individual components, including especially the assessment of bearings and concentricity. Moreover, it allows detecting any changes in the state of the machine foundation. Mechanical vibrations are a very fast oscillatory motion of a specific mechanical system around the axis of its equilibrium and they are caused by the inertia of individual elements (reciprocating motion) or unbalance of rotating parts.The measurement of vibrations and oscillations is usually performed at the place where the machine is installed, with the use of various types of meters and sensors. Detailed tests reveal the displacements, velocities and accelerations of vibrations, while the frequency analysis of vibrations enables signal distribution into components. These data, together with the machine's operating parameters and its design, enable maintenance engineers to assign specific machine elements to individual spectrum components or to determine the machine’s states at a specific moment.
Such data is obtained by maintenance engineers thanks to the most modern generation of vibrometers, which constitute an important part of the TME product portfolio. The diversity in this group of meters is large and the devices themselves are so interesting that they deserve to be discussed and presented in more detail.
Vibration sensors, meters and analysers, usually called vibrometers, are portable devices which, thanks to various vibration detection methods – based on accelerometers or laser technology – measure the vibration level of a machine or its components. It is a reflection of the technical condition, and its thorough analysis makes it possible to diagnose the defects or wear of bearings, determine possible misalignment or asymmetry, and detect plays or throws. Constant or regularly repeated monitoring of vibration levels also makes it possible to predict failures before they occur.
Currently produced mobile vibrometers often resemble typical meters of electrical parameters: they have a similar body and are equipped with similar multi-line LCD displays. The most modern vibrometers combine the functions of a meter and vibration analyser, tachometer and balancer. They allow the maintenance engineers to perform general vibration measurements, assess the condition of the machine according to specific standards, check the wear of rolling bearings, determine the temperature and speed of the rotating element, as well as balance the rotor. During these processes, a modern vibrometer saves all the data in its memory and then sends it to a PC or a mobile device equipped with dedicated analysis and archiving software.
The most common vibrometers on the market belong to one of two types: vibrometers based on accelerometers or laser vibrometers. The former measure both displacement and acceleration, allowing the sensor to be mounted directly on the machine and indicate the magnitude of the vibration with high accuracy. The sensor can be an integral part of a vibrometer – or it can be a small accessory connected to the meter with a special connector and then fixed with a magnet on the machine, which works in tight spaces. Their key element – the accelerometer, or rather the transducer used in it – may belong to one of three different types: piezoelectric, piezoresistive or capacitive. In piezoelectric transducers, the crystals generate an electrical charge under mechanical load, in piezoelectric transducers the electrical resistance is measured, while in capacitive transducers the changes in capacitance are measured depending on the position of the moving electrodes.
Laser vibrometers employ completely different technology. Their operation consists in detecting and analysing laser radiation, which is dispersed on a vibrating part of the machine. A well-known phenomenon called the Doppler effect is applied here: when an object vibrates, then the frequency of light changes and the measurement of the difference in frequencies becomes the basis for calculating the speed of movement of the examined object, the amplitude of its vibrations or acceleration.
Vibrometers can be used to measure vibrations on any part of a machine, but the recommendations of the relevant standards indicate above all the most sensitive places, such as bearing housings, because there the level of vibrations and amplitudes of forces emerging there are usually the highest. However, it is important to remember about the basic principles of reliable vibration measurements: to perform the test always in the same conditions, in the same place (point) and to apply a vibration meter in the same way. Without this, it is difficult to compare the results between individual measurements, to assess the possible deterioration of the machine in comparison with previous results, not to mention to identify the trend of changes in the vibration level over time.
The most typical application of vibrometers is to assess the degree of wear of bearings. Such wear always increases as a result of the radial or axial play or as a result of defects within the cage that holds the rolling elements. As the condition of the bearing continues to deteriorate, the resistance to motion and friction and increases, and so does temperature, which further intensifies the degradation of the component. Another typical application of vibrometers is the diagnostics of gears, which generate excessive vibrations and signal their wear due to the pitting effect. It consists in the formation of microcracks at the contact point of two metallic surfaces, their breaking up and finally leads to material particles chipping out from the surface of the gear. The third most common application of vibrometers is the detection of unbalance of a machine rotor or a component. A rotor that is not correctly balanced attempts to rotate around the central mass axis, but is limited by bearings. As a result of centrifugal forces, this discrepancy is manifested by characteristic vibrations, which may eventually lead to damage to the bearings or the rotor itself.
The question of the vibration sensor, first of all, boils down to deciding whether a contact vibrometer is sufficient for proper diagnosis or whether the circumstances require the use of contactless laser vibrometers. In the case of the former, it may turn out that a typical portable vibrometer is not always able to reach all measurement points due to the tightness of the machine’s construction – then, it is necessary to choose a model equipped with connectable sensors and probes, easy to mount even in very tight spaces. Further selection issues are determined by the capabilities of the device itself and the expectations of maintenance engineers: the simplest models provide measurement information, but if more in-depth (3D) analysis is required, then more advanced models from renowned manufacturers should be used.
The first of vibrometers available in our catalogue is a device by UNI-T, marked with the UT312 symbol. This handy, lightweight – only 0.25 kg – and the portable meter is a basic device for measuring mechanical displacement, acceleration and linear speed and presenting the results on an LCD display. The measuring range of this vibrometer is, respectively: 199.9 m/s2, 199.9 m/s and 1999 µm with an accuracy of +/- 5% and a frequency of 0.01 - 1.5 kHz. Additional functionalities include freezing the display (Auto-Hold), low battery indication (type 9V 6F22) and automatic switching off.
The Fluke brand – represented in the TME’s offer by three models – doesn't need to be introduced to anyone. The TME catalogue includes two compact and portable twin models – Fluke 805 and 805 FC – and one larger model – Fluke 810, which performs more advanced analysis based on measurements taken. Below, you will find a short description of Fluke vibrometers available at TME.
Fluke 805 is a vibration meter for maintenance engineers who require repeatable measurements of vibration and inspection of the condition of bearings according to the scale of significance. It weighs only 400 g, requires 2 AA batteries and is quite compact thanks to its dimensions: 58 x 241 x 71 mm. This device is used to measure mechanical vibrations and other parameters, such as the bearing condition and temperature (contactless), and, thanks to the combination of the vibration sensor and measuring probe, it ensures repeatable results at different set-up angles or pressures. The four steps on the significance scale represent the results in a clear way through text messages (good, satisfactory, unsatisfactory and unacceptable). The vibration sensor reads a wide range of frequencies (10 to 1000 Hz and 4000 to 20 000 Hz), covering most types of machines, and is supported by an intuitive user interface.
The measurement procedure itself is further facilitated by a colour diode (green, red) and on-screen instructions which indicate the correct pressure required to make a proper measurement. The Fluke 805 also supports an external sensor for hard-to-reach places.
The twin 805 FC model is the same vibrometer, but it can be linked to the Fluke Connect® application, allowing the technician to share test results and validate subsequent actions when working in the field. This feature also allows a specialist to manage resource-based measurements anywhere, using the EquipmentLogTM history log, and to connect wirelessly with other technicians (equipped with devices with the same functionality) to show them the real-time situation in a video call.
Other key parameters of this model are as follows: acceleration measurement range from 0.01 to 50G, temperature measurement (infrared) range from -20 to +200°C and operating temperature from -20 to +50°C.
An important functionality of this device is the ability to export data (memory can store up to 3500 measurements) via USB and visualize trends in Microsoft® Excel by means of built-in templates. The “Crest Factor+” functionality enables relatively accurate assessment of the bearing condition by means of direct measurements using the sensor tip in the range from 4000 to 20 000 Hz. The audio output for direct listening to the bearing noise and the torch function for illumination of measurement points in low light conditions are also worth appreciating.
Fluke 810 is a more advanced diagnostic tool that provides immediate measurement results. Thanks to Fluke’s unique technology, this equipment quickly recognizes mechanical problems and suggests priorities for their resolution. In fact, Fluke 810 reports faults in the machine under test right from the first measurement, without any history of previous measurements, and then assesses the severity of these faults and makes recommendations for corrective actions. This model is quite compact given its weight (18.6 x 7 x 26.7 cm with a weight of 1.9 kg), but at the same time much bigger than the handy Fluke 805 vibrometer.
The reliability of measurements performed by this model is enhanced by a laser tachometer for precise measurements of machine speed and a 3-axis accelerometer. Space for storing the results is provided by an internal 2 GB memory, which can be extended and from which the results can be transferred to a computer via Mini USB 2.0. Another useful feature is the possibility of performing an auto-test of the device, which results in a longer working time and high efficiency of the meter. Fluke 810 analyzes equipment such as motors, fans, belt and chain drives, gearboxes, connections, pumps and compressors of various types, as well as spindles and close-coupled machines, among others. Its most important technical parameters are as follows: automatic change of ranges, operating band from 2 Hz to 20 kHz (sampling at 51 kHz), complex functions of digital signal processing (anti-aliasing filter, high-pass filter, decimation, overlapping, windowing, Fast Fourier Transform (FFT) and averaging). It is worth mentioning that the device is powered by a 2.55 Ah Li-Ion battery, which ensures really long-lasting work on one charge. Of course, Fluke 810 – like all vibrometers of this class – provides concrete figures, which describe the actual condition of the component, allows to recognize and locate the most frequent mechanical faults (type of failure – bearings, misalignment, asymmetry, play), offers a 4-level fault severity scale (in 805 / 805 FC models) and presents specific countermeasures or advice in real time (useful for new users).
Vibrometers by Extech, a company belonging to American FLIR corporation, are represented by six portable models of vibrometers, two of which – SDL 800 and SDL 800-NIST – are twin products, the latter being a version that is enriched with the NIST manufacturer calibration certificate. The other models are: classic EX 407860, VB 300 (one of the smallest models on the market), a little bigger VB 400 and medium-sized VB 450.
EX 407860 and VB 300 models are particularly noteworthy, the former being a classic vibrometer with an LCD display and HOLD function – it be operated with one hand. This model operates in three ranges and measures acceleration (200 m/s2), speed (200 m/s2) and displacement (2 mm) thanks to a remote, magnetic vibration sensor mounted on a 1-meter cable. The frequency range spans from 10 Hz to 1 kHz and the basic measurement accuracy is +/- 5%. The equipment has an option to record Max and Min values, which can be recalled from the internal memory. The RMS or peak measurement mode, and the built-in RS-232 communication interface with optional data distribution software are also worth a mention here. EX 407860 is offered in a suitcase together with a battery, belt pouch and a measuring probe.
Unlike EX 407860, the miniature VB 300 model is in fact not so much a vibrometer, but rather a 3-axis accelerometer with a data logger, equipped with a USB interface for data transfer. An inexperienced user may even think it is a flash drive, but of course, it is a completely different device.
This vibration recorder – probably the smallest one available on the market – analyses vibrations in the FTT frequency domain, records more than 100,000 samples per axis (X, Y, Z or any combination thereof) or more than 150,000 regular samples, offers adjustable sampling times (50 ms-24 h) and the manual or programmable start mode. The kit offered by TME includes an accelerometer, a small Li-Ion battery and Windows® compatible data analysis software.
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